1. Field of the Invention
The present invention relates to a primary-side driving backlight circuit for a Liquid Crystal Display (LCD) panel, which employs a single isolation device, transferring the conducting pulse signal output by the Pulse Width Modulation (PWM) controller located on the secondary-side to the primary-side, to achieve the isolation request for safety for the secondary-side, and further employs the High/Low side driver, to drive the power switch element, so as to provide better circuit simplification and functions than conventional methods, which is particularly significant in terms of circuit simplification and functions for full-bridge based topology.
2. Description of Related Art
As illustrated in FIG. 1, a half-bridge based primary-side driving backlight power supply of prior art is shown. AC inputs L and N are rectified by a bridge rectifier BD1 and pass through a power factor correcting circuit (consisting of capacitor C10, transformer T1, transistor Q10, resistor R1, PFC controller 8, diode D10, capacitor C11), generating a stable DC input power source VIN to supply required energy for outputting. When the push-pull PWM controller 10 on the secondary-side starts to operate and the output terminals DRV1 and DRV2 of the push-pull PWM controller 10 begin to send out pulse width signals, the input terminals of the isolation driving transformer T3 is coupled to the output terminals DRV1 and DRV2 of the push-pull PWM controller 10 and one of the two output winding sets of the isolation driving transformer T3 has the same polarity with the input winding, while the other output winding has the opposite polarity. Therefore, the isolation driving transformer T3 generates positive and negative pulse driving signals, whereas the driving circuits 14a, 14b are respectively responsible for filtering the positive/negative pulse signals and outputting modulated positive/negative pulse signals, so as to avoid abnormal actions in the power switches Q11, Q12. When the output terminal DRV1 of the push-pull PWM controller 10 presents high level, the power switch Q11 is conducting, and when output terminal DRV1 becomes low, the power switch Q11 is cutoff. For the other output terminal DRV2 of the push-pull PWM controller 10, when it becomes high level, the power switch Q12 is conducting, and when output terminal DRV2 becomes low, the power switch Q12 accordingly is cutoff, which situation repeats again and again, keeping the entire system stable. During the dead time of half-bridge based output pulse, current continuously flows through the body diodes of the power switches Q11, Q12. Because the current flowing through the half-bridge based power switch is twice as much as the current in the full-bridge based one, the half-bridge based is less efficient than the full-bridge based, while the advantage of half-bridge based system is its simple circuit arrangement.
As illustrated in FIG. 2, a primary-side driving backlight power source supply of prior art is shown. The secondary-side circuit and actions are almost identical, and the output terminals DRV1 and DRV2 of the push-pull PWM controller are respectively coupled to the isolation driving transformers T3 and T4; the isolation driving transformers T3 and T4 are components of the exact same structure, which individually has a set of input winding, and two sets of output windings with the same polarity as the input winding. In the isolation driving transformers T3 and T4 shown in FIG. 2, each end of the input winding thereof is coupled to a different output of the push-pull PWM controller, such that the positive/negative pulse signals on the input winding of the isolation driving transformers T3 and T4 are exactly opposite, and the positive/negative pulse signals on the output winding of the isolation driving transformers T3 and T4 are also exactly opposite, but the output of the same isolation driving transformer is designed to have positive/negative pulse signal of the same polarity. In this way, the power switches Q11, Q14 are simultaneously controlled to be conductive, while the power switches Q12, Q13 are also simultaneously controlled to be conductive in another conducting time; meanwhile, the driving circuit 14a, 14b, 14c, 14d coupled to controlled ends of each power switches are responsible for filtering the positive/negative pulse signals, so as to avoid abnormal switch action. The dead time of the full-bridge based primary-side driving backlight power source supply of prior art shown in FIG. 2 and the dead time of the push-pull PWM controller 10 are identical, but in case that the conducting time is designed to be small, then the dead time will become larger. During the dead time, the current only flows through the body diode with larger voltage fall, thus causing higher power loss.
The prior art primary-side driving backlight power source supply, by using a transformer, converts the signals on the output terminals DRV1, DRV2 of the push-pull PWM controller, and transfers the converted signals to the driving circuits DRIVER1 (ie. 14a), DRIVER2 (ie. 14b) to achieve isolation request for safety. However, it takes more energy to drive a transformer, the driving energy requirement of signal on the output terminals DRV1 and DRV2 of the push-pull PWM controller is accordingly higher; therefore it is more demanding on the driving capability of the push-pull PWM controller; besides, the cost and size of the transformer are more significant, causing the increase of system cost.